Microsoft has set Oct. 31 as the end of sales of new consumer-grade Windows 7 PCs, but for now has left open the do-not-sell-after-this-date for business machines.

On the site where it posts such policies, Microsoft now notes that Oct. 31, 2014, is the end-of-sales date for new PCs equipped with Windows 7 Home Basic, Home Premium or Ultimate. All three are consumer-oriented versions of Windows 7; Home Premium has been the overwhelming choice of OEMs (original equipment manufacturers) for consumer systems.

Microsoft's practice, first defined in 2010, is to stop selling an older operating system in retail one year after the launch of its successor, and halt delivery of the previous Windows edition to OEMs two years after a new version launches. The company shipped Windows 8, Windows 7's replacement, in October 2012.

The setting of a deadline for consumer Windows 7 PCs followed a glitch last year when Microsoft named the same Oct. 31 date for all Windows 7 PCs, but then quickly retracted the posting, claiming that the notification had been posted "in error."

But while it has established an end-of-sales date for consumer PCs with Windows 7 pre-installed, Microsoft has yet to do the same for business PCs.

Microsoft will give a one-year warning before it demands that OEMs stop selling PCs with Windows 7 Professional, the commercial-quality version. Under that rule, Microsoft will allow computer makers such as Lenovo, HP and Dell to continue selling PCs with Windows 7 Professional until at least February 2015.

It's likely that the extension will be much longer.

Windows 7 has become the standard version for businesses, which have spurned Windows 8, largely because of its two-user interface (UI) model, which they consider disruptive to productivity and a needless cost that would require employee retraining.

Most analysts believe that Windows 7 will remain the most popular Microsoft operating system deployed by companies for years to come.

"There's a good chance that enterprises will stay on Windows 7 as long as possible," said Gartner analyst Michael Silver in an October 2013 interview. If his prediction turns out to be accurate, Windows 7 may reprise the stubborn persistence of Windows XP, the nearly-13-year-old OS that Microsoft will retire in April.

Even after Windows 8's launch, Windows 7's user share, a rough measurement of the prevalence of the OS on operational machines, has continued to grow. From October 2012 to January 2014, Windows 7's user share increased nearly 3 percentage points, representing a 6% gain during that period, according to data from analytics company Net Applications.

Some of Windows 7's gains certainly came at the expense of Windows XP, which has fallen more than 11 percentage points, a 28% decline, since October 2012 as users abandoned the old OS.

By making Windows 7 available, Microsoft and its OEMs not only continue to serve customers who want the OS, but make sure that new PC sales do not slump even more dramatically than they have already.

Consumer PC sales have plummeted -- last month Microsoft said sales of consumer-grade Windows licenses fell 20% in the December quarter compared to the same period the year before -- while the Redmond, Wash. company's business line of operating systems grew 12% year-over-year. In effect, enterprise spending kept PC shipments from tanking even more than the 10% contraction the industry experienced in 2013.

Extending Windows 7 Professional's availability on new hardware will also give Microsoft breathing room to continue its retreat from Windows 8's radical shift to a touch-first, tile-based UI, and to roll out a successor that caters even more to customers who rely on keyboard and mouse.

Retail sales of Windows 7 by Microsoft to distributors and customers were officially halted as of Oct. 31, 2013, but that deadline has been meaningless, as online retailers have continued to sell packaged copies, sometimes for years, by restocking through distributors who squirreled away older editions.

As of Saturday, for example, Amazon.com had a plentiful supply of various versions of Windows 7 available, as did technology specialist Newegg.com. The former also listed copies of Windows Vista and even Windows XP for sale through partners.

Even after Microsoft pulls the plug on Windows 7, there will be ways to circumvent the shut-down. Windows 8.1 Pro, the more expensive of the two public editions, includes "downgrade" rights that allow PC owners to legally install an older OS. OEMs and system builders can also use downgrade rights to sell a Windows 8.1 Pro-licensed system, but factory-downgrade it to Windows 7 Professional before it ships.

And enterprises with volume license agreements will never be at risk of losing access to Windows 7, as they are granted downgrade rights as part of those agreements, and so will be able to purchase, say, Windows 8.1 or Windows 9 PCs in 2015 or 2016, then re-image the machines with Windows 7.

The end-of-sales dates for Windows 7 are not linked in any way to the support schedule for the 2009 operating system. Microsoft will provide free non-security bug fixes and vulnerability patches for Windows 7 until Jan. 13, 2015 -- called "mainstream support" -- and follow that with a five-year stretch of "extended support" during which it will ship free security updates until Jan. 14, 2020.